Electrophotographic photoreceptor are known which include a material using an inorganic photoconductive substance and a material using an organic photoconductive substance. Such photoreceptor using an organic photoconductive substance have been increasingly used in recent years because of the advantages that they cause less pollution of the environment, their productivity is high and production costs are low.
Such organic photoconductive substances for generating an electric charge on absorbing visible light have a disadvantage that the electric charge retention is poor. On the other hand, a substance having a good electric charge retention and excellent film-forming properties generally has a disadvantage that it does not always exhibit photoconductivity due to visible light.
In order to overcome the above disadvantages, a laminate-type electrophotographic photoreceptor comprising an electric charge generating layer which generates an electric charge on absorbing visible light and an electric charge transporting layer which transports the electric charge have been developed. In this laminated structure, an electric charge generating material and an electric charge transporting material can be chosen independently, and they can be chosen from a wide variety of materials. Representative examples of electric charge generating materials include polycyclic quinone pigments, perillene pigments, indigo pigments, bisbenzoimidazole pigments, quinacridone pigments, phthalocyanine pigments, mono-azo pigments, di-azo pigments, tri-azo pigments, poly-azo pigments and the like. Representative examples of electric charge transporting materials include amine compounds, hydrazone compounds, pyrazoline compounds, oxazole compounds, oxadiazole compounds, stibene compounds, carbazole compounds and the like.
In preparing an electrophotographic photoreceptor using the above materials, it is necessary to select a material which is satisfactory in all characteristics, e.g. electrophotographic characteristics such as sensitivity, electric receiving potential, electric potential retention properties, electric potential stability, residual electric potential, spectral characteristics and the like, and use characteristics such as strength, durability, anti-pollution properties and the like.
Of the above characteristics, sensitivity is the most important for an electrophtographic photoreceptor, and thus it is still desired to increase the sensitivity. The sensitivity is mainly determined by a combination of an electric charge generating material and an electric charge transporting material. However, the sensitivity also varies with preparation conditions of the electric charge generating layer and the electric charge transporting layer, the method of preparation of components, and so forth.
The sensitivity can be increased to a certain extent by increasing the thickness of the electric charge generating layer and/or the electric charge transporting layer. Increasing the thickness of the layers, however, is limited because of resulting changes in physical properties other than the sensitivity and because of limitations of production techniques and so forth. To increase the sensitivity, it is also effective to increase the proportion of the electric charge generating material in the electric charge generating layer. However, this is also limited by changes of physical properties other than the sensitivity and limitations of production techniques. It has, therefore, been more desirable to increase the sensitivity with specified types of an electric charge generating material and an electric charge transporting material.